In B-mode medical ultrasound imaging, an ultrasound scanner transmits an ultrasound signal into a patient and detects the intensity of the signal reflected from different depths. The scanner thus provides an image of structures within the body. Conventional Doppler imaging goes one step further and detects the velocity of moving tissues and fluids in the direction of the transmitted ultrasound signal. The conventional Doppler ultrasound machine employs two imaging modalities for measuring velocity: color flow imaging and strip Doppler imaging. Ultrasound scanners typically implement strip Doppler imaging in one of three different modes: PW or pulse wave imaging, HPRF or high pulse repetition frequency imaging, and CW or continuous wave imaging. According to the pulse Doppler technique, the ultrasound scanner places a range gate over the region of interest, and interrogates the region with multiple ultrasound pulses. The scanner samples the returning echoes and determines velocities using the Doppler principle. The velocity of the target is calculated and displayed based upon the sampled echoes.
HPRF imaging is similar to PW imaging. The difference is that the imaging system places and receives echoes from multiple range gates within the body. In HPRF mode, echoes from prior transmissions reflected from deep structures within the body are received at the same time as echoes from shallow structures. The system samples the composite echo from all range gates, and displays the frequency spectrum (converted to velocity scale) of the composite echo.
In continuous Doppler imaging, the scanner transmits and receives continuous ultrasound signals to and from the body. The scanner calculates the Doppler frequency spectrum according to continuous-time techniques.
All three strip Doppler imaging modes employ the same display mechanism. The Doppler frequency spectrum is plotted along the ordinate against time along the abscissa. In practice, the frequency axis is converted into a velocity axis using the Doppler equation. Velocity of the target toward and away from the transducer is displayed along the positive and negative halves of the ordinate, respectively. The system modulates the brightness of the pixels in each frequency bin to display the energy of that bin.
In addition to displaying the Doppler frequency spectrum graphically, the ultrasound system typically converts the Doppler frequency information to audio signals. The system distinguishes between the velocity of the target toward and away from the transducer by sending it to different speakers.
Variance is an inherent attribute of all velocity estimates obtained from a Doppler system. The width of the Doppler spectrum is one measure of variance. The spectral width obtained using any of the methods outlined above is due to one or more of many factors, including flow characteristics, signal bandwidth, transit time effects and variance in parameter estimates.
The sensitivity of the Doppler method in measuring the velocity of the target is best if the velocity is parallel to the ultrasound propagation direction. The velocity estimates tend to have a large variance if the angle between the propagation direction and the velocity is larger than sixty degrees. In those situations, velocity estimates are not good indicators of the presence of flow.
Further, the amount of flow is not readily apparent from the conventional strip display mechanism discussed above. As an example, if there are two plug flows that are identical in velocity and different in flow volume, this difference would be difficult to perceive, especially if the difference in volume is small. In that instance, both flows would cause the same frequency or velocity band to be bright and the resulting small difference in brightness would be the only indication of the difference between the two flows.
Audio processing of Doppler information also has the same disadvantages listed above. If the velocity estimates experience a wide variance, the audio output will be noisy. Furthermore, the strength of the audio output is no indication of the amount of flow in conventional systems.
To overcome these problems, some ultrasound systems employ Doppler techniques to measure reflected energy or power. For example, U.S. Pat. No. 5,243,987, issued to Shiba, describes an ultrasound system for measuring the backscattering power of blood. Shiba employs high pass filtering and thresholding to eliminate reflections from slow-moving structures, such as tissue. The Shiba system displays the intensity of the echo signal by varying the brightness of a gray scale image or the hue of a color display, or as a three dimensional plot of Doppler spectrum against frequency and time. U.S. Pat. No. 5,285,788, issued to Arenson, and assigned to the assignee of the present invention, provides a Doppler tissue imaging (DTI) system that produces a color Doppler image of moving tissue representing estimates of velocity and reflected energy.
U.S. Pat. No. 5,014,710, issued to Maslak, and assigned to the assignee of the present invention, describes a color Doppler imaging system that processes Doppler-shifted echoes into blood flow information, including velocity, variance and power. Further, U.S. patent application Ser. No. 08/691,204, entitled "Imaging Modality Showing Energy and Velocity," and assigned to the assignee of the present invention, discloses a color Doppler imaging system having a mixed mode in which luminance is a function of the product of the velocity and the energy of the echo signal. All of the patents, applications and other references discussed herein are incorporated by reference herein.
One disadvantage of color display techniques is that the display frame rate is relatively slow with respect to the cardiac cycle. Further, because of the relatively low number of samples used to compute Doppler shift, color systems exhibit a relatively poor signal-to-noise ratio.
The present invention overcomes these disadvantages by providing more flexibility in the diagnostic parameters available in ultrasound machines implementing other display modes.
Further, the invention has potential advantages over another imaging technique--integrated backscatter imaging. This imaging technique is generally used for estimating the echo-density over a region of integration. B-mode information is integrated over a region and displayed instead of the usual echo intensity. The principle behind this technique is the assumption that the energy reflected from a region is related to the density of the reflecting medium at that location. A corollary mode currently exists whereby the integrated backscatter information can be obtained via Doppler processing through the use of Doppler tissue imaging (DTI) in the energy mode.